The present invention relates to the general field of turboprops including at least one set of variable-pitch fan blades. The invention relates more particularly to controlling the pitch of fan blades in a two-propeller airplane turboprop.
In known manner, a two-propeller airplane turboprop comprises a turbine having two contrarotating rotors, each driving a set of unducted fan blades. By way of example, reference may be made to document GB 2 129 502 that describes various embodiments of such a turboprop. The invention applies more particularly to turboprops in which the propellers are mounted at the rear.
In this type of turboprop, the pitch of the fan blades in each set (i.e. the angular orientation of each blade) constitutes a parameter that is used for controlling the thrust of the turboprop. For this purpose, a known solution for controlling the pitch of the fan blades in a given set has recourse to actuators that are positioned on the rotary portions of the turboprop.
Such control devices are particularly suited for the set of blades that is situated furthest towards the rear of the turboprop. The stator portion of the turboprop is remote from this set, such that it is difficult to install actuators that are secured to a structural element that is stationary. By way of example, reference may be made to patent document EP 1 921 325 that describes a device in which each blade is coupled for pitch adjustment to an actuator member of an annular actuator carried by the rotary casing of the turboprop.
Nevertheless, controlling the pitch of fan blades by means of actuators carried by the rotary portions of the turboprop presents drawbacks. In particular, feeding hydraulic fluid to actuators that are moving in rotation gives rise to manifest practical problems of implementation. Furthermore, the behavior of hydraulic fluid when subjected to centrifugal force is not well known and of uncertain reliability.